Proto proposal clean up

This proposal to change the protocol environment does the following:

• remove some parallel operators (from Error_monad and Lwt) to reduce the likelihood of protocols having undeterminism
• remove one catch-all pattern exposed through the Option module (the pattern was unused anyway)
• remove MBytes (which has been deprecated in favour of Bytes/TzEndian)

Note that these commits are built on top of a commit that introduces an environment V2. I don't know whether the transition from V1 to V2 is supposed to be that sudden or whether there is a different plan in place. The V2-introduction commit might need to be swapped out for something else.